1. Field of the Invention
The present invention relates to telemetry systems for communicating information from a downhole location to a surface location, and, more particularly, to a method of determining channel characteristics of a mechanical telemetry system such as a mud pulse telemetry system. The term “mechanical” as used herein relates to telemetry governed by, or in accordance with the principles of mechanics, including acoustics and fluid mechanics.
2. Description of the Related Art
Drilling fluid telemetry systems, generally referred to as mud pulse systems, are particularly adapted for telemetry of information from the bottom of a borehole to the surface of the earth during oil well drilling operations. The information telemetered often includes, but is not limited to, parameters of pressure, temperature, direction and deviation of the well bore. Other parameters include logging data such as resistivity of the various layers, sonic density, porosity, induction, self-potential and pressure gradients. This information is critical to efficiency in the drilling operation.
MWD Telemetry is required to link the downhole MWD components to the surface MWD components in real-time, and to handle most drilling related operations without breaking stride. The system to support this is quite complex, with both downhole and surface components that operate in step.
In any telemetry system there is a transmitter and a receiver. In MWD Telemetry the transmitter and receiver technologies are often different if information is being up-linked or down-linked. In up-linking, the transmitter is commonly referred to as the Mud-Pulser (or more simply the Pulser) and is an MWD tool in the BHA that can generate pressure fluctuations in the mud stream. The surface receiver system consists of sensors that measure the pressure fluctuations and/or flow fluctuations, and signal processing modules that interpret these measurements.
Down-linking is achieved by either periodically varying the flow-rate of the mud in the system or by periodically varying the rotation rate of the drillstring. In the first case, the flow rate is controlled using a bypass-actuator and controller, and the signal is received in the downhole MWD system using a sensor that is affected by either flow or pressure. In the second case, the surface rotary speed is controlled manually, and the signal is received using a sensor that is affected.
For uplink telemetry, a suitable pulser is described in U.S. Pat. No. 6,626,253 to Hahn et al., having the same assignee as the present application and the contents of which are fully incorporated herein by reference. Described in Hahn '253 is an anti-plugging oscillating shear valve system for generating pressure fluctuations in a flowing drilling fluid comprising a stationary stator and an oscillating rotor, both with axial flow passages. The rotor oscillates in close proximity to the stator, at least partially blocking the flow through the stator and generating oscillating pressure pulses. The rotor passes through two zero speed positions during each cycle, facilitating rapid changes in signal phase, frequency, and/or amplitude facilitating enhanced data encoding.
U.S. Pat. No. RE38,567 to Gruenhagen et al., having the same assignee as the present invention and the contents of which are fully incorporated herein by reference, and U.S. Pat. No. 5,113,379 to Scherbatskoy teach methods of downlink telemetry in which flow rate is controlled using a bypass-actuator and controller.
There are two groups of signals that are used in the transmission of information: baseband and passband. The transmission of information in a sequence of pressure pulses is known as baseband signaling. If the pulses are further modulated by a carrier signal, which shifts the transmission bandwidth higher in frequency, then this is known as passband signaling. The particular examples taught in Gruenhagen are baseband signaling. Hahn '253, on the other hand, also teaches the use passband signaling with different types of modulation techniques such as Frequency Shift Keying (FSK), Phase Shift Keying (PSK) and Amplitude Shift Keying (ASK). U.S. patent application Ser. No. 10/223,169 of Hahn et al., having the same assignee as the present invention and the contents of which are fully incorporated herein by reference, further teaches use of multivalent coding, and chirp signals for uplink telemetry. It should be noted that passband signaling can also be used for downlink telemetry with the method of Gruenhagen. Scherbatksoy discloses a system in which a narrow band signal is modulated by square pulses representative of the data to be transmitted.
Baseband signaling is affected by torque noise and the stick-slip behavior of the drillstring. By using passband signaling we can move the signal clear of this noise source. The passband frequency bandwidth, however, is also not free of noise sources: for example, pump noise can play a major role in distorting the signal. But if the contaminating noise is generated by the pumps then we can independently quantify it and filter it out from our measurements. In summary, by using passband signaling we gain control over the frequency bandwidth occupied by the information carrying signal, and can better cancel noise that may be distorting this signal.
In passband filtering, one of the central issues is the selection of frequencies at which the signal is to be transmitted. The present invention addresses this issue.
Another problem addressed in this invention is not limited to borehole telemetry applications and occurs wherever mechanical vibrators are used to generate swept frequency signals. The problem is that of harmonic distortion wherein the mechanical vibrator generates one or more harmonics of the fundamental frequencies. The received signals are cross-correlated with the reference sweep signal to get an impulse response of the channel. In the presence of harmonics, the processed signal may be severely affected by harmonic distortion. A method for attenuating harmonic correlation noise caused by harmonic energy output from seismic vibrators was developed by Reitsch as disclosed in U.S. Pat. No. 4,042,910. The method includes the step of generating a plurality of sweep signals in series and with the phase of each succeeding sweep signal being shifted relative to the previous one by a predetermined phase angle that is a fraction of 2π. The generated signals are separately recorded and transformed by inverse phase shifting before being added or stacked in a conventional manner. The generation of multiple sweeps is time-consuming and the phase encoding adds to the complexity of the system. The problem of harmonic distortion also occurs (but is not addressed) in US20030151975 of Thomann et al. in seismic-while-drilling applications. The problem also occurs in cross-well tomography where swept frequency sources are used. The present invention addresses the problem of harmonic distortion without the need for multiple sweeps or phase encoding ot the signals.